As is known, a longwall mining machine traverses a coal seam and has two rotating cutter heads at opposite ends of the machine which cut and remove the coal and direct it onto a face conveyor extending along the path of travel of the mining machine. In certain types of longwall installations, the mining machine is caused to traverse the coal seam by means of a driven gear which engages a gear rack extending along the face side of the aforesaid conveyor.
In U.S. Pat. No. 4,166,652, a longwall mining machine is disclosed for use in low coal seams which includes a machine support rack along the working face side and a guide bar along the stowing side. The two cutting drums of the machine are carried at the machine ends and are vertically adjustable by means of adjustable arms positioned above the working face wall of the aforesaid conveyor. The mining machine bears on only one track positioned on the working face side of the conveyor and is guided by the aforesaid guide bar along the stowing side of the conveyor. The machine is caused to move along the coal seam by means of two driving gears which engage a gear rack on the working face side of the conveyor.
In a longwall mining machine of the type described above, it is necessary for the center of gravity of the machine to be situated as closely as possible to the guiding track on the working face side of the conveyor. This is necessary so as not to impair the stability of the conveyor. Those parts of the mining machine nearest the face conveyor are, therefore, disposed directly above the conveyor. Specifically, the cutter drum support arms are disposed above the face conveyor or above the gear rack which is disposed on the working face side of the conveyor. As a result, longwall mining machines of this kind can be used only in seams with a minimum thickness of approximately 3 feet, because the lowest pivoting position of the support arms is restricted by the presence of the gear rack on the working face side of the conveyor beneath the support arms. In the lowest position of the leading pivoting support arm, the cutter drum which it carries must still be capable of cutting free the coal or other material extending to the bottom edge of the face conveyor over its entire length. As a result, this lower position of the support arm defines the minimum cutter drum diameter which, in turn, defines the minimum thickness of a seam on which such a longwall mining machine can be used.
A reduction of the cutter drum diameter, which would enable a longwall mining machine to operate in seams less than 3 feet in thickness, can be achieved only if it becomes possible to extend the adjustability of the drum support arm toward the mine floor such that the drum diameter can be reduced to the dimension defined by the top edge of the machine.
In U.S. Pat. No. 4,082,361 a gear rack is disclosed for guiding a longwall mining machine which comprises two elongated strips or plates between which extend gear rack pins designed to engage a driving gear on a longwall mining machine. Such a gear rack, however, limits the extent to which the cutter drum support arms can be lowered and, hence, limits their use in low coal seams.
British Pat. No. 1,367,055 discloses a gear rack for longwall mining machines comprising a quadrilateral section, subdivided into individual portions, which is provided with bores disposed at specific pitch distances from each other. Each bore is intended to receive one bolt; while all bolts have a length which exceeds the width of the quadrilateral profiled section and are supported so as to be longitudinally slideable in the bores. The bolts are pushed through the bores by the longwall mining machine with the aid of a shoe which slides on the side of the quadrilateral profiled section which is remote from the driving gear of the mining machine. The bolt ends which project from the bores on the other side of the quadrilateral profiled section serve to engage the mining machine driving gear which rolls along the aforesaid gear rack.
The gear racks described in the foregoing publications both have relatively large widths. In the gear rack disclosed in U.S. Pat. No. 4,082,361, the width is defined by the two lateral strips or plates which carry the gear rack pins between them. In the type of gear rack disclosed in the British patent specification, the width is defined by the width of the quadrilateral profiled section which must be dimensioned such that the bolts, which are slideably supported therein and project beyond the width of the profiled section, continue to be retained in their working positions in which they project from the profiled section by more than the width of the driving gear. As a result, both types of gear racks limit the downward movement of the drum-cutter support arms.